1. Field of the Invention
The present invention relates to a transmission type screen, to which image light is projected from a rear side of the screen as seen from a viewer, to thereby display an image, and more particularly, to a transmission type screen in which image degradation caused by speckles is reduced, an image display method for the transmission type screen, and a projection display apparatus including the transmission type screen.
2. Description of the Related Art
A projection display apparatus is an apparatus for displaying an image using a combination of a Fresnel lens screen and a diffusion sheet (diffusion layer). Unlike a cathode ray tube (CRT) display and a plasma display panel (PDP), the projection display apparatus is a non-self-luminous display apparatus.
The projection display apparatus includes: an illumination optical system serving as a projector, for guiding light from a light source in a predetermined direction; a light valve illuminated with the light guided by the illumination optical system, for adjusting the amount of light based on an image signal to form an image; and a projection optical system for enlarging and projecting the image formed by the light valve to a screen.
The projection display apparatuses are broadly categorized into a rear projection type display apparatus for projecting image light to a screen from a rear side of the screen as seen from a viewer and a front projection type display apparatus for projecting image light to the screen from a front side of the screen as seen from the viewer. A transmission type screen used for the rear projection type display apparatus of the former type includes: a Fresnel lens screen for bending image light from a projector to a viewer side; and an image display element for forming an image of the image light from the Fresnel lens screen and expanding the light beam at a set dispersion angle.
A Fresnel lens is generally formed to have a lens period smaller than a projection pixel size (for example, 1/10 of pixel size), and thus has a very small size even in the thickness direction (thickness including prism portion thickness is, for example, several hundred μm). Therefore, in order to hold the Fresnel lens, a base having a thickness range of approximately 1 mm to 5 mm is required.
In many cases, the base is made of a resin such as polymethyl methacrylate (PMMA), methyl methacrylate-styrene (MS), methyl methacrylate-butadiene-styrene (MBS), or polycarbonate (PC), or a glass.
In many cases, the Fresnel lens is made of, for example, a light curable resin and directly formed on a base. An element including the Fresnel lens and the base is called a Fresnel lens screen.
An image display element includes at least light diffusion means and a base. The light diffusion means uses surface unevenness larger than a wavelength of light (visible light range of 380 nm to 780 nm) or a refractive index difference between a diffusion layer base material and a fine particle larger than the wavelength of light.
A case in which a structure having a fluctuation larger than the wavelength of light is illuminated with light which is enlarged by the projection optical system to obtain large spatial coherence is considered. In this case, even when the light source is a lamp light source having small temporal coherence, a large number of bright and dark spots (glare) are randomly recognized. The bright and dark spots are generally called speckles (technically, subjective speckles) or scintillation, which causes image degradation.
As measures against such speckles (or scintillation), a technology of providing orthogonal polarization for each region using a diffusion member and a retardation plate having different optical axis directions for different regions has been proposed (see, for example, Japanese Patent Application Laid-open No. 2008-165058).
However, a conventional technology has the following problems.
In the conventional technology as disclosed in Japanese Patent Application Laid-open No. 2008-165058, the retardation plate having different optical axis directions for different regions is required to be located immediately in front of the screen. Therefore, a large patterned retardation plate equal in size to the screen is necessary, and hence there is a problem that manufacturing is difficult and a cost is high.
Further, the different regions are periodically provided, and hence there is also a problem that it is likely to generate moire fringes with a projection pixel, a Fresnel lens, or another micro-lens.